1. Field of the Invention
The present invention relates to a technique for performing a streak correction on image data of a document.
2. Description of the Related Art
Image reading apparatuses using an automatic document feeder are known. The image reading apparatuses using the automatic document feeder convey documents placed on a document tray one by one, and at the same time, read an image of the conveyed document at a reading position. These image reading apparatuses read an image at the unchanged reading position, and, therefore, are subject to generation of a streak in a read image when dust, such as paper dust, dirt, and motes, carried by the document conveyance is attached on the reading position.
An image reading apparatus discussed in Japanese Patent Application Laid-Open No. 2005-45462 acquires read data when a document does not pass through a reading position yet, read data of a leading edge portion and a trailing edge portion of the document, and read data of a main area (an area other than the leading edge portion and the trailing edge portion) of the document. Then, if all of the pieces of read data contain pixels having the same density at a certain pixel position along a conveyance direction of the document (hereinafter referred to as a sub-scanning direction), the image reading apparatus detects a pixel at the certain pixel position as a streak pixel based on dust attached on the reading position. Hereinafter, in the present disclosure, a streak pixel based on dust will be simply referred to as a streak pixel.
According to the apparatus discussed in Japanese Patent Application Laid-Open No. 2005-45462, dust hidden at the back side of the document does not appear as a streak in the read image when the leading edge portion and the trailing edge portion of the document pass through the reading position. Therefore, a matching result cannot be acquired from a comparison between the density when the document does not pass through the reading position, and the density when the document passes through the reading position. As a result, such dust is not detected as a streak pixel. On the other hand, dust unhidden at the back side of the document appears as a streak in the read image, whereby a matching result can be acquired from the comparison between the density when the document does not pass through the reading position, and the density when the document passes through the reading position. As a result, the dust is detected as a streak pixel.